In the formation of one or more required holes in a hydroformed metal part, it is well known that the holes can be formed with a punch in a piercing operation on the part. While the part remains in the hydroforming dies following its hydroforming to the required shape and with the support of the internal hydroforming pressure. For example, there is shown in FIG. 1 of the accompanying drawings a prior art device that performs such a piercing operation. With such operation and the device for accomplishing it also being referred to herein as “hydropiercing” and “in-die hydropiercing device”, respectively.
In the exemplary prior art device in FIG. 1, the piercing is performed on a hydroformed part A while the part remains in the hydroforming dies B and C in which it was formed and the hydroforming pressure is maintained therein. The piercing is typically performed by a flat-faced punch D that is received in a ring-shaped die button E that is mounted in the die cavity surface F of one of the hydroforming dies in a position aligned with where the hole is required in the part. In this case the hole is required in the upper side of the part A and therefore the die button E is located in the die cavity surface of the upper die C. The punch D and a central bore in the die button E in which the punch is received have a cylindrical shape corresponding to that of the hole required in the part and which is typically a right-circular, cylindrical shape to produce a circular hole but can also be of some other cylindrical configuration or shape such as oval, square and rectangular.
The punch D is operated by a hydraulic cylinder G and is initially positioned thereby so that the face H of the punch together with the face I of the die button form a continuation of the surrounding die cavity surface for the hydroforming of the part A from a piece of tubular metal stock. Wherein a suitable liquid is supplied to the interior of the piece at a sufficiently high pressure such as about 10,000 psi to form the part outwardly against the die cavity surface. The punch D following formation of the part is then extended by the hydraulic cylinder G as shown and with the support of the hydroforming pressure in the part acting outwardly on the wall of the part about the die button face I pierces a required hole J in the part. Whereby a slug K is separated from the wall of the part in the formation of the hole, settles to the bottom of the part and must be removed later. Moreover, as the punch D pierces the part, the hydroforming fluid with such a punch is prone to leak out of the hole past the punch causing a significant drop in internal pressure. Which can result in a collapse of the wall of the part adjacent the hole such as to the configuration shown in phantom-lines. Moreover, such leakage makes it difficult, if not impossible, to punch more than one hole in the part using similar punch devices whereas more holes may be required in the part and could be punched simultaneously if none of the punch devices caused significant leakage during their piercing operation.
The quest for efficient sealing during piercing and efficient slug removal are but two of the goals faced in expanding the use of in-die hydropiercing. As larger holes are being required which has necessitated larger diameter, more costly, hydraulic cylinders in order to deliver the larger piercing forces required. And not only are larger, more costly, hydraulic cylinders required for the larger holes, they typically must have a longer stroke for both relatively small and large holes where the punch is designed to also form an inward extrusion in the wall of the part surrounding the hole that will effectively seal against the punch to prevent leakage. Such as disclosed in U.S. Pat. No. 6,658,908 that is assigned to the assignee of this invention. Or the hydraulic cylinder may require an unusually large stroke simply to push the larger slugs at least to the side and out of the way of the hole. Moreover, such larger hydraulic cylinders are not only more costly; they require significantly larger packaging space in the hydroforming apparatus and significantly larger hydraulic fluid flow in order to operate.